Hydrofluorocarbons (HFCs) and in particular hydrofluoroolefins (HFOs), such as 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf), are compounds known for their properties of refrigerants and heat-exchange fluids, extinguishers, propellants, foaming agents, blowing agents, gaseous dielectrics, polymerization medium or monomer, support fluids, agents for abrasives, drying agents and fluids for energy production units. Unlike CFCs and HCFCs, which are potentially dangerous to the ozone layer, HFOs do not comprise chlorine and thus do not present a problem for the ozone layer.
1,2,3,3,3-Pentafluoropropene (HFO-1225ye) is a synthetic intermediate in the manufacture of 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf).
The majority of the processes for the manufacture of hydrofluoroolefins involve a dehydrohalogenation reaction. Thus, the document WO 03/027051 describes a process for the manufacture of fluoroolefins of formula CF3CY═CXnHp, in which X represents a hydrogen atom or a halogen atom chosen from fluorine, chlorine, bromine or iodine and n and p are integers and can independently take the value zero, 1 or 2, provided that (n+p)=2, which comprises bringing a compound of formula CF3C(R1aR2b)C(R3cR4d), with R1, R2, R3 and R4 independently representing a hydrogen atom or a halogen atom chosen from fluorine, chlorine, bromine or iodine, provided that at least one of R1, R2, R3 and R4 is a halogen atom and that at least one hydrogen atom and one halogen atom are situated on adjacent carbon atoms, a and b being able independently to take the value zero, 1 or 2, provided that (a+b)=2, and c and d being able independently to take the value zero, 1, 2 or 3, provided that (c+d)=3, into contact with at least one alkali metal hydroxide in the presence of a phase transfer catalyst.
This document teaches, in Example 2, that, in the absence of a phase transfer catalyst, there is no reaction when 1,1,1,3,3-pentafluoropropane (HFC-245fa) is brought into contact with a 50% by weight aqueous potassium hydroxide (KOH) solution at ambient temperature and under pressure for 24 hours.
In addition, this document teaches a reaction temperature of between −20° C. and 80° C.
The document WO 2008/075017 illustrates the dehydrofluorination reaction of 1,1,1,2,3,3-hexafluoropropane (HFC-236ea) to give 1,2,3,3,3-pentafluoropropene (HFO-1225ye) at 150° C. in the presence of a 50% by weight aqueous KOH solution. In the absence of a phase transfer catalyst, the conversion after 3 and a half hours is 57.8% and the selectivity for HFO-1225ye is 52.4% (Test 1). In the presence of a phase transfer catalyst, this conversion is reached after only 2.5 hours and the selectivity is virtually unchanged (Test 4). As indicated in Table 2 of this document, it is necessary to use an organic solvent in order to increase the selectivity for HFO-1225ye.
WO 2007/056194 describes the preparation of HFO-1234yf by dehydro-fluorination of 1,1,1,2,3-pentafluoropropane (HFC-245eb) either with an aqueous KOH solution or in the gas phase in the presence of a catalyst, in particular over a catalyst based on nickel, carbon or a combination of these.
The document Knunyants et al., Journal of the USSR Academy of Sciences, Chemistry Department, “Fluoroolefin Reactions”, Report 13, “Catalytic Hydrogenation of Perfluoroolefins”, 1960, clearly describes various chemical reactions on fluorinated compounds. This document describes the dehydrofluorination of 1,1,1,2,3,3-hexafluoropropane (236ea) by passing through a suspension of KOH powder in dibutyl ether, to produce 1,2,3,3,3-pentafluoro-1-propene (HFO-1225ye) with a yield of only 60%. This document also describes the dehydrofluorination of 1,1,1,2,3-pentafluoropropane (HFC-245eb) to give 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf) by passing into a suspension of KOH powder in dibutyl ether with a yield of only 70%.
Furthermore, FIG. 2 on page 51 of Part 2 of the nouveau traité de chimie minerale [New Treatise on Inorganic Chemistry] by P. Pascal, 1963 Ed., shows the appearance of the liquid/solid equilibria of the water and potassium hydroxide system and the measurements are collated in the Table on page 52.
A process for the manufacture of a compound of formula (I) CF3—CF═CHX in which X represents a hydrogen or fluorine atom, comprising at least one stage of dehydrofluorination of a compound of formula (II) CF3—CHF—CHFX, with a very good selectivity and/or high yield, has now been found.
A subject-matter of the present invention is thus a process for the manufacture of a compound of formula (I) CF3—CF═CHX in which X represents a hydrogen or fluorine atom, comprising at least one stage of dehydrofluorination of a compound of formula (II) CF3—CHF—CHFX during which the compound of formula (II) is brought into contact with a mixture composed of water and of potassium hydroxide in which the potassium hydroxide is present in an amount of between 58 and 86% by weight, preferably of between 58 and 69% by weight and advantageously of between 60 and 66% by weight, the said mixture being maintained at a temperature of between 125 and 180° C.
Preferably, the temperature of the water and hydroxide mixture is between 145 and 180° C., and advantageously between 152 and 165° C.
According to one embodiment of the invention, the mixture of water and of hydroxide can be obtained from hydrates of formula KOH.x.H2O, x being between 1 and 2 inclusive.
The dehydrofluorination stage can be carried out in any type of reactor known to a person skilled in the art. Use may be made of a stirred reactor, a static mixer, a reactive column or a nozzle or, very simply, the compound of formula (II) can be bubbled into the said mixture of water and of potassium hydroxide present in a vessel.
In addition to the dehydrofluorination stages, the process comprises a stage of separation of the compound of formula (I), optionally followed by a purification stage.
The operation can be carried out continuously, semicontinuously or batchwise at atmospheric pressure or under pressure, preferably less than 2 bar absolute.
The Applicant Company has observed that the rate of dehydrofluorination of the compound of formula (II) is very high with the mixture of water and of hydroxide as described above and thus particularly advantageous for carrying out the process according to the present invention continuously.
According to a preferred embodiment of the invention, the compound of formula (II) and the mixture composed of water and of hydroxide as defined above are introduced continuously into a reactor, initially charged with this mixture, maintained at the abovementioned temperature and a stream comprising the compound of formula (I) is continuously withdrawn from the gas phase of the reactor. The potassium fluoride formed as by-product can be withdrawn continuously or batchwise from the liquid reaction medium, for example by filtration. The concentration of water in the reaction medium can be kept constant by continuous evaporation of the water formed by the reaction.
When the operation is carried out batchwise or semicontinuously, the KOH/compound of formula (II) molar ratio involved is generally between 2 and 100, preferably between 3 and 20.
The process is very particularly suitable for the manufacture of 2,3,3,3-tetra-fluoropropene by dehydrofluorination of 1,2,3,3,3-pentafluoropropane.
It is also suitable for the manufacture of 1,2,3,3,3-pentafluoropropene by dehydrofluorination of 1,1,1,2,3,3-hexafluoropropane. The 1,2,3,3,3-pentafluoropropene can be in the form of the Z and/or E isomer.
The Applicant Company has discovered that the vinyl trifluoride by-product can be reduced in the case of the dehydrofluorination of HFC-236ea. This by-product becomes negligible when the dehydrofluorination temperature is between 125 and 145° C. and the KOH is present at between 58 and 69% by weight, preferably between 60 and 66% by weight, in the water-KOH mixture.
It can be advantageous to use an inert gas, preferably nitrogen or hydrogen, in the dehydrofluorination stage.
Depending on the conversion desired, the dehydrohalogenation reaction can be carried out in several stages, optionally using several reactors in series, preferably in two stages, optionally using two reactors in series.
The process according to the present invention has the advantage of resulting in high yields, even in the absence of phase transfer catalyst and/or of organic solvent.